nosedive

Short definition from Wiktionary

Noun

1. 1
   A headfirst fall or jump.
2. 2
   A drop in altitude with the nose of the craft angled downwards.
3. 3
   A rapid fall, e.g. in price or value.

Verb

1. 1
   To dive down in a steep angle.
2. 2
   To perform a rapid fall in price or value.